As is known in the gas turbine art, compressor surge can occur as a consequence of stall conditions on a sufficient number of compressor blades. Blade stall is known to result from various conditions, including: severe acceleration (variously referred to as "jam" or "bodie"), during sideslip or skid (when in evasive action); as a consequence of turbulent air which upsets conditions at the engine air inlet; or from a violent afterburner light-off. These perturbations cause a sufficient change in the velocity of gas flow through the engine in contrast with the rotational velocity of the compressor blades, for a given compressor blade angle, so that the resultant angle of attack causes airfoil stall. When sufficient number of blades are stalled, a surge (a detectable, violet engine event) may occur. A mild surge may simply result in a momentary pressure drop and flow reversal of the gas stream which is self-recoverable. A more severe surge may result in multiple surge cycles which may be self-recoverable or may require control action to assist recovery. Under certain conditions, surges can result in rotating stall and airflow stagnation, in which the stall condition has so upset the compression process that sufficient energy for recovery is not available, without some external action being taken.
In the art, a variety of means are known for detecting surge/stall. For instance, devices which sense radial or upstream/downstream pressure ratios are disclosed in U.S. Pat. Nos. 3,858,625 and 4,103,544. Other devices sense stall as a function of compressor discharge pressure (or combustor inlet pressure) as in U.S. Pat. No. Re. 29,667. Still others utilize combinations of the fuel control schedule with temperature and/or pressure events in the engine, such as in U.S. Pat. Nos. 4,060,979, 4,060,980 and 4,117,688. A more currently common manner of detecting stall/surge is a ratio of temperature to rotor speed, as disclosed in U.S. Pat. Nos. 4,108,926 and 4,137,710. The early detection of stall is desirous in order to take corrective action to avoid multiple cycle surges or more severe surge effects, referred to as rotating stall stagnation. For instance, in U.S. Pat. No. Re. 29,667, stall sensed by compressor discharge pressure is utilized to open compressor bleeds, thereby unchoking the compressor and aiding it in recovery from the stall condition. Other corrective actions, such as reducing fuel flow and adjusting the angle of some of the compressor vanes (in an axial flow gas turbine), are also known.
In a commonly owned, copending U.S. patent application entitled "Electrostatic Gas Turbine Surge/Stall Detection", Ser. No. 454,121, filed contemporaneously herewith by St. Jacques et al, the early sensing of stall stagnation, and discriminating stall stagnation from surge is described as useful in providing corrective action for early recovery from stall stagnation.
In the aforementioned St. Jacques et al. application, rapid detection of any stall condition is achieved by utilizing a voltage-biased electrostatic probe at a dilution air (cooling or mixing air) inlet of the combustor (burner can); the probe normally is bathed simply in unburned air, but when reverse flow occurs, flame streams out of the combustor can past the electrostatic probe and provides significantly increased conductivity between the probe tip and the walls of the engine. This results in an electrical signal which is timed precisely to the reverse flow out of the combustor can.
One problem with the electrostatic surge/stall detector of the St. Jacques et al. application is that the probe is ineffective in the absence of flame. In some stall events, particularly where heavy de-rich (low fuel) is being utilized, no flame is sensed by the electrostatic probe. In other cases, it is possible for stall stagnation to occur to a sufficient degree to result in a flameout, whereby the flame-responsive surge/stall detector of the aforementioned application would provide an indication of recovery from stall stagnation correction, when such is not the case.